31 Immune System and Disease

31 Immune System and Disease

KEY CONCEPTS

31.1 Pathogens and Human Illness

Germs cause many diseases in humans.

31.2 Immune System

The immune system consists of organs, cells, and molecules that fight infections.

31.3 Immune Responses

The immune system has many responses to pathogens and foreign cells.

31.4 Immunity and Technology

Living in a clean environment and building immunity help keep a person healthy.

31.5 Overreactions of the Immune System

An overactive immune system can make the body very unhealthy.

31.6 Diseases That Weaken the Immune System

When the immune system is weakened, the body cannot fight off diseases.

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938 Unit 9: Human Biology

31 Immune System and Disease

KEY CONCEPTS

31.1 Pathogens and Human Illness

Germs cause many diseases in humans.

31.2 Immune System

The immune system consists of organs, cells, and molecules that fight infections.

31.3 Immune Responses

The immune system has many responses to pathogens and foreign cells.

31.4 Immunity and Technology

Living in a clean environment and building immunity help keep a person healthy.

31.5 Overreactions of the Immune System

An overactive immune system can make the body very unhealthy.

31.6 Diseases That Weaken the Immune System

When the immune system is weakened, the body cannot fight off diseases.

BIOLOGY

BIOLOGY

View animated chapter

Keep current with biology news.

concepts.

? Featured stories

? Vaccines and Active Immunity ? News feeds

? Destroy the Invaders

? Bio Bytes

RESOURCE CENTER

Get more information on ? Immune Response ? Allergies ? Autoimmune Disorders

938 Unit 9: Human Biology

How do your cells fight off

invaders?

You do not get sick every time diseasecausing germs invade your body. Sometimes white blood cells, like the one in blue above, attack and destroy invaders without your feeling ill. Other times, you get sick because germs, such as the purple E. coli above, start winning. Fortunately, a healthy immune system can overpower many different types of germs--even when the germs temporarily gain the upper hand.

colored SEM; magnification 4000

Connecting CONCEPTS

Plants Spots on a plant's leaves are evidence that the plant's immune system is fighting an infection. When a germ invades a plant, the infected cells release chemicals that kill the neighboring plant cells. With the surrounding cells dead, the germ is isolated and cannot infect the rest of the plant. Here, an English oak tree leaf has responded to a parasitic infection by causing its own cells around the infection sites to die (orange).

Chapter 31: Immune System and Disease 939

31.1

Pathogens and Human

Illness

KEY CONCEPT Germs cause many diseases in humans.

MAIN IDEAS

? Germ theory states that microscopic particles cause certain diseases. ? There are different types of pathogens. ? Pathogens can enter the body in different ways.

VOCABULARY germ theory, p. 941 pathogen, p. 941 vector, p. 944

10.d Students know there are important differences between bacteria and viruses with respect to their requirements for growth and replication, the body's primary defenses against bacterial and viral infections, and effective treatments of these infections.

Connect Diseases caused by germs, such as the E. coli bacteria on the previous

page, can be fatal. From 1330 to 1352, the bacteria that caused the Black Death killed 43 million people worldwide, or 13 percent of the population at the time. In 1918, a viral disease called the Spanish flu killed between 20 and 50 million people worldwide, or as much as 3 percent of the population. Because diseases can have devastating effects, scientists become concerned whenever a new disease such as HIV, SARS, or avian flu appears.

MAIN IDEA

Germ theory states that microscopic particles cause certain diseases.

A disease can be either infectious or noninfectious. Infectious diseases, such as flu and polio, can be passed from one person to another because infectious diseases are caused by germs. In contrast, cancer and heart disease are noninfectious diseases. These diseases are called noninfectious because a sick person cannot pass the disease to, or infect, a healthy person. Noninfectious diseases are not caused by germs; they result from a person's genetics or lifestyle.

FIGURE 31.1 History of Medicine

Most modern understanding about diseases occurred after Pasteur's germ theory.

B.C. 7000

Spirits Ancient societies drill holes in people's heads to release the evil spirits believed to cause disease.

B.C. A.D.

B.C. 460?B.C. 377

Humors Greek physician Hippocrates hypothesizes that fluids, called humors, cause disease.

A.D. 1330?1352

Herbal treatments People

use incense in an attempt to

cure those with the Black Death, caused by bacteria transmitted by rats' fleas.

(LM; magnification 15)

1857

Germ theory Louis Pasteur hypothesizes that disease is caused by small "animals."

1400

1600

1400?1600

Anatomy People begin to study anatomy. This drawing was made in the Middle East in 1555.

1800

1865

Antiseptic technique Joseph Lister finds that cleaning his surgical tools reduces patients' infections.

940 Unit 9: Human Biology

On the other hand, infectious diseases can be passed from one person to another because infectious diseases are caused by germs.

Today, it seems obvious that some germs cause infectious disease, but this concept is only a little more than 100 years old. It was not until the 1850s that French scientist Louis Pasteur helped make the connection between microorganisms and disease. His theory, called the germ theory of disease, proposed that specific microorganisms caused diseases. These disease-causing agents are called pathogens. Pasteur hypothesized that if pathogens were eliminated from the body, a person would not get sick.

Pasteur's germ theory led to rapid advances in our understanding of disease, as shown in FIGURE 31.1. But at the time, germ theory was not immediately accepted. It took the work of two other scientists to bring about the complete acceptance of Pasteur's germ theory.

Between 1861 and 1865, about half of British surgeon Joseph Lister's patients died from infections after otherwise successful operations. After hearing Pasteur's germ theory, Lister began using a weak acid to clean his operating tools and his patients' wounds before surgery. The number of his patients who died from infection dropped dramatically to near zero.

Meanwhile, German scientist Robert Koch found that he could make a healthy animal sick by injecting it with pathogens from a sick animal. From his experiments, he concluded that four conditions must be met before one can say that a certain pathogen causes a disease. These conditions are called Koch's postulates.

? The pathogen thought to cause the disease must be present in every case in which the disease is found.

? The pathogen must be isolated and grown outside the body in a pure, uncontaminated culture.

? Healthy animals infected with the pure culture must develop the disease. ? The pathogen must be re-isolated and cultured from the newly sick ani-

mals and must be identical to the original pathogen.

Contrast How is germ theory different from earlier theories about disease?

TAKING NOTES Use a main idea diagram to study germ theory of disease.

germ theory

1883

Koch's postulates Robert Koch finds four conditions that prove a pathogen causes a disease.

1928

Antibiotics Sir Alexander Fleming discovers penicillin.

1900

1900s

Applying antiseptic technique Cities around the world start treating drinking water with chlorine, reducing the cases of cholera.

1955

Polio vaccine Jonas Salk's vaccine against polio becomes available. The disease is eliminated in the U.S. in 1994.

2002

New diseases First cases of SARS, a disease that affects the respiratory system, spring up in China.

2000

2005

Polio comeback Worldwide efforts increase to vaccinate people against polio, and the polio virus reemerges in fewer than ten people in the U.S.

Chapter 31: Immune System and Disease 941

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